The CVD technology is widely utilized in IC fabricating processes. For a CVD process needing low pressure like a LPCVD or high-density plasma CVD (HDP-CVD) process, a vacuum pump is installed to the deposition chamber. For a traditional pump like a rotary pump can merely make a low pressure in the order of mTorr, it is usually used in combination with a turbo pump as the required pressure is in the order of Torr.
An exemplary structure of a HDP-CVD system for depositing silicon oxide with a rough pump for rough pumping and a turbo pump is shown in FIG. 1. The chamber 100 has therein a wafer chuck 110 and gas inlets 120. The rough pump 140 is coupled with the chamber 100 via a rough line 130, the turbo pump 150 is coupled with the same by a gate valve, and the rough pump 140 is contiguous with the turbo pump 150 via a foreline 160. In the CVD process, the throttle valve of the rough line 130 is closed but the gate valve is opened, and the rough pump 140 and the turbo pump 150 are both switched on. The unreacted gases and the by-products are conducted from the chamber 100 and through the gate valve, the turbo pump 150, the foreline 160 and the rough pump 140 and then exhausted to a pipeline (not shown).
Because silicon oxide film also accumulates in the chamber 100 to be particle source, the chamber 100 has to be cleaned after being used to process a plurality of wafers. In such a clean process, a halogen-containing gas and its reactive radical capable of reacting with silicon oxide to form gaseous products, such as NF3 and fluorine radical, is delivered through the gas inlets 120 into the chamber 100 with the throttle valve of the rough line 130 opened, the gate valve closed and the rough pump 140 switched on, as shown in FIG. 1. However, even with such a chamber clean step, the reduction in particle number and the mean wafer between clean (MWBC) success ratio are still limited due to the unstable non-lot and inline particle performance.
The particles are suspected to come from the turbo pump 150 because the particles accumulated in the turbo pump 150 may be streamed back to the chamber 100 during a pressure transition of the film formation process. The silicon oxide particles inside the turbo pump 150 cannot be effectively removed by NF3 and fluorine radical because the gate valve is closed in the chamber clean step and the chamber 100 is non-contiguous with the turbo pump 150.